i cant really think of any free tools which does what your asking, but u can download a trial version and use it to find out what you need....

this will exactly what you need, show all devices, bandwidth, create alerts based on bandwidth, breakdown of bandwidth by port etc

netfort languardian

NetFort LANGuardian is software for investigating and monitoring activity on your network. Use it for network forensics, troubleshooting bandwidth issues, and keeping track of Web activity and Windows file share activity

There are free things like capsa, nagios, catci, some solarwinds apps...

To use anything that isn't integrated into your router requires one of two things: SNMP trap on the router sent to the app, or port mirroring in the switch to send all traffic that is going to the router to the port where your app lives.

I use mikrotik for bandwidth management, check who is consuming how much bandwidth and restricting the bandwidth usage for particular computer or vlan.http://www.mikrotik.com/
It can be installed in PC or mikrotik routerboards itself.